It targeted PC World because it's the only major store that offers to fix computers even if not originally purchased at one of its stores.

The magazine's researchers deliberately "broke" computers in order to assess the level of service at various repair centres across the UK.

On half of the computers they simply deleted a Windows start up file to prevent the operating system from er, starting. On the remaining computers they loosened a hard disk cable.

The "faulty" computers were then taken along to a total of 20 PC repairers and the results of the investigation showed that the independent stores performed much better than big boys PC World.

Diagnosis of the faults at PC World ranged from reinstalling Windows to advising the "customer" to replace their computer. In some instances the reinstallation led to personal data being lost as work had not been backed up.

Repair costs also varied wildly at PC World between £69.99 and a staggering £350.

On the whole, independent stores provided a better service according to the report, with more accurate diagnosis and repair costs typically lower than PC World.

In the report, Computing Which? advised its readers not to go to PC World if they need to get their computer repaired. The magazine's editor said: "We think that they [PC World] should overhaul their training – they could learn a lot from independent retailers."

A PC World spokeswoman said the Which? report was not representative of the level of service and expertise offered by the chain.

"PC World's skilled engineers have fixed more than two million PC problems for customers in the last year, and in nine of 10 PC repairs there was no requirement to fit a spare part," she said.

She says the Which? visits preceeded "an extensive overhaul of our national support operation, including our services for customers in-store".

"We'd like to stress that our pricing for all services is transparent, and we always discuss the potential cost with customers prior to carrying out any installation or repair service. In many, if not most cases, the actual cost is considerably lower than the initial estimate." ®